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1 FIRST RISK-BASED SCREENING ON A SPANISH PORTFOLIO OF 20 DAMS OWNED BY THE DUERO RIVER AUTHORITY Ignacio Escuder, Universidad Politecnica de Valencia, SPAIN Manuel G. de Membrillera, Universidad Politecnica de Valencia, SPAIN Pedro Moreno, Confederacion Hidrografica del Duero, SPAIN Oscar Perez, Ofiteco, SPAIN Liana Ardiles, Confederacion Hidrografica del Duero, SPAIN Esther Jenaro, Confederacion Hidrografica del Duero, SPAIN ABSTRACT Society is constantly evolving and the fact of the matter is that demand for transparency in decision-making has turned out to be a highly topical reality. It is in this context where the growing complexity that involves dam safety management can be found out. Due to historical and technical reasons, dam safety has traditionally been based on the so called traditional standards-based approach, where risk is tackled by following recognised defensive design measures, taking extreme values for the loads, conservatively safe values for resistance variables and applying safety coefficients. However, in a climate of growing public scrutiny, the traditional standards-based approach, by itself, is becoming increasingly limited to handle a single dam or a portfolio of dams in allocating limited resources for their operation. A qualitative application (screening level) has been carried out by first time on a Spanish portfolio of 20 dams, owned and operated by the Duero River Authority (Confederacion Hidrografica del Duero). In a very simplified manner, the analysis involves potential failure modes, load scenarios, probabilities related to the system response and consideration of life loss and economic consequences. Results show the importance of developing a Dam Safety Program based upon risk analysis and failure mode thinking in the realm of Spanish current dam safety practice. 1

2 Introduction The article describes the risk analysis methodology, at screening level, as it has been customized and applied to a portfolio of 20 dams owned and operated by the Duero River Basin Authority (CHD) in Spain. In particular, the procedure of USBR (5) has been adapted and complemented with some of the works of the edams group at Universidad Politecnica de Valencia (Spain), who collaborates with the engineering firm OFITECO, SA in the development of the contract entitled Redaccion de Planes de Emergencia e Informes de Seguridad de las presas de Camporredondo, Compuerto, Requejada, Cervera y Aguilar de Campoo. First part of the article leads with the contextualization of the methodology in a more general risk assessment framework as applied to dam safety. Second part is devoted to specific objectives and fundamentals of the screening procedure. Finally, the results of the particular application to the portfolio of dams are given and analyzed. Contextualization The need to implement a risk assessment technique as a decision support tool for dam safety management emerged in the early nineties in some of the most developed countries in the world, such as those described in ANCOLD (1), Bedford et al (2), Bowles et al (3), USBR (6), FERC (7), G.Membrillera et al (8), Hartford et al (10) or ICOLD (11). Some of the main reasons for this need are listed below: a) Aging of dam structures (majority of dams being older than 30 years and a great percentage over 50 years in operation) including a gap between present-day good practice and the one followed when many existing dams were designed and constructed. b) An increasing demand for safety for populations and properties located downstream. c) A growing request for better justification of funding all aspects of dam safety programs. d) Shifts to risk management approaches in business and regulation rather than an exclusive reliance on traditional engineering standards. e) A growing backlog of dam safety improvements and the need to prioritise them to achieve the fastest rate of risk reduction. f) The difficulties in constructing new dams due to environmental and social factors. g) The need to optimize water resources system management as well as to increase storage capacity in response to a continuously growing water supply demand and an apparent increase in extreme meteorological events (such as severe droughts and floods). In this context of dam maintenance requirements, improving operating procedures and increasing regulation, estimating different types of risk (structural, operational, etc.) becomes a crucial need. Even more, the identification of tolerable risk levels (both related to the damreservoir system and water supply) should be an available tool for decision makers. 2

4 Nowadays in Spain, classical dam safety calculations based on a pseudo-probabilistic load hypothesis and partial safety factors cannot satisfy these goals. The above-mentioned pseudo-probabilistic analysis implies that, for both flood and earthquake loads, a deterministic assumption is made about the water level that is used in dam safety calculations. Thus, even though the occurrence of floods and earthquakes are recognised as random processes, this is not taken into account when applying this approach. In addition, partial safety factors lead to acceptance/non acceptance criteria but cannot directly be related to a probability of failure. Recognizing all the above mentioned limitations, the CHD decided to develop a risk based complementary safety program that, as shown in the flowchart of Figure 1, incorporates the information coming from the normal activities in any dam safety program such as inspections, instrumentation, emergency plans etc. and provides an added value by dealing explicitly with risk. First step in such development consisted in a screening level analysis for a portfolio of 20 dams. Risk analysis at screening level As mentioned in the precedent chapter, risk based dam safety programs have the main objective of identifying is current risk are tolerable and, in any case, select the appropriate risk control measures. The assessment of such tolerability is accomplished by means of the so called Risk Analysis and Risk Evaluation activities, as shown in Figure 2. More in detail, the screening level deals only with a number of the risk analysis tasks (shadowed in Figure 2), namely the load scenarios, failure modes, system response, failure probability and failure consequences. Figure 2. Screening contextualization on the global risk management framework (shadowed). From: G.Membrillera (9). 4

5 Gestión de Riesgo Declaración de Riesgo Control de Riesgo Análisis de Riesgo Escenarios de solicitación Modos de fallo Respuesta de la presa Probabilidad de rotura Consecuencias de la rotura Estimación de Riesgo Evaluación de Riesgo Legislación Normativa Guías Técnicas Soslayar el riesgo:solución alternativa Reducir riesgo: vigilancia e implementación del P.E.M. Reevaluar periódicamente estado del arte Cuál es el riesgo de la presa? Es aceptable el riesgo? Se encuentra el riesgo dentro de límites aceptables? Comparación entre la situación actual de la presa y aquella en la que debería encontrarse Acciones para controlar el riesgo y llevarlo hasta límites aceptables Main objectives The screening analysis is aimed to score a portfolio of dams, in a general and simplified manner, in terms of their potential failure risk. Consequently, the final goal is provide an objective information to help in the organization, planning and resource assignment to dam safety programs by identifying those dams with a higher level of risk. It is however essential that, despite the simplicity of the procedure, a relevant mistake (such as not accounting a significant risk) is not made as it may imply that the dam would not receive the due attention and its safety can be jeopardized. Input data and other requeriments The input data should be collected from information gathered in safety related normal activities such as past visual inspections, field investigations or instrumentation records. In the case of Spain, mandatory documents according to the current legislation (14) such as the Operating Rules, Emergency Action Plan and Dam Safety Review reports are of great help to complete all 5

6 needed data. Nevertheless, to properly complete the score of all dams, engineering judgment from an experience technician is unavoidable. In order to evaluate the required resources for any organization to complete a screening analysis, according to the acquired experience by CHD, the activity that consumes most of the time is collecting the proper information. If such information is provided by previous documented works, and experience engineer should be able to complete the score in no more than few hours per dam. Theoretical fundamentals Taking into an account that the screening analysis main objective is to make a homogeneous comparison on potential risks associated to a portfolio of dams, risk should be herein properly defined. Risk (CSA, 1997 Hartford et al (10) and ICOLD(11)) is a measure of the probability and severity of an adverse effect to life, health, property or environment. In the general case, risk is estimated by the combined impact of all triplets of scenario, probability of occurrence and the associated consequence. In the special case, average risk is estimated by the mathematical expectation of the consequences of an adverse event occurring. Expressing this in terms of an equation: R [P(solicitaciones) P(rotura solicitaciones) P(consecuencias rotura,solicitaciones)] (ec. 1) where R is risk, P(X) the conditional probability estimated for the event X, and the expression P(X Y) is the probability of the event X given the event Y. As shown in Table 1, adapted from USBR (5) and where the score system can be checked in full detail, the screening analysis deals with risk associated to three general load scenarios (hydrologic, seismic and normal operation), adding all three partial results to get an overall index. The obtained Failure Index combines scenarios with the two first components of the risk equation: loads at column A and system response at column B. Table 1. Screening summary result table for every dam D. A. B. C. E. Load Loss Load Response Filure Risk Scenarios of Life Factor Factor Index Index Factor F. Population at Risk G. Socio- Economic Index H. Consequence Factor Static Hydrologic Seismic Operations, Maintenance, and Safety Totals 6

7 Consistently with the definition of risk provided previously, the failure index is multiplied by a Potential Life Loss Factor that accounts for consequences of the dam failure, giving the Risk Index as final result. In addition, a measure of potential social and economic impacts are reflected through use of the Socio-Economic Index. Also, for the purposes of the screening, the term social is assumed to grossly include cultural and environmental consequences. Customized components As properly referred before, the features of the scoring methodology can be fully checked at USBR (5). However, some remarks on particular customizations or specific recommendations suitable for the Spanish context are listed bellow: Concerning the seismic load factor factor, it is exclusively linked with the so called Basic horizontal acceleration (a b ) as referred in the current Spanish seismic code (13). Concerning Worksheet G (Operations, Maintenance, and Safety), the minimum discharge in absence of flood can be evaluated from scenario A1 in the Spanish Emergency Action Plans. Concerning Worksheet H (Loss of Life Factor), a simplification from the McClelland and Graham methodology (4) was adopted, making use of the mandatory information that should be included in the Spanish Emergency Action Plans and some procedures implemented by Triana (15). Figure 3 summarizes all factors involved in the procedure. Figure 3. General scheme for potential loss of life estimation. Fuente: Jonkman et al (12). %población evacuada: FE Efectos físicos: (x) Función de pérdidas: F M %población no evacuada: (1-F (x)) E Mortandad: F (x) M Poblaciónenriesgo: PER (x) Población no evacuada: (1-F (x)) PER (x) E Pérdidadevidas humanas: N(x) Población evacuada: F (x) PER (x) E Supervivientes: PER (x) - N(x) 7

8 Finally, a Worksheet I (Consequence Qualitative Index) has been added by the authors to the existing methodology so that a qualitative estimation of material damages can be incorporated to the over all judgment. These consequences can be classified as A (high), M (medium) or B (low) according to the reservoir uses (Flood Control, Water supply and irrigation, Hydropower Recreational and others) and the magnitude of the impact on population, economy and environment. 4. SCREENING APPLICATION TO A PORTFOLIO OF 20 DAMS According to the previously described theoretical principles and practical procedures, the following graphs show the results of the screening application to a portfolio of 20 dams owned and operated by the CHD in Spain (Figs. 4 to 8). Figure 4. Risk Index summary of results 120,000 Indices de riesgo en las presas de la C.H. del Duero 100,000 80,000 Puntuación 60,000 40,000 20,000 0 Aguilar Requejada Cervera Compuerto Camporredondo Barrios Benamarías Collado 1 Collado 2 Linares Pontón San José Selga Valdesamario Villameca Cogotas Arlanzón Cuerda del Pozo Uzquiza Porma Riaño Explotación ordinaria Hidrológico Sísmico 8

14 (14) Ministerio de Obras Públicas, Transportes y Medio Ambiente (1996). Reglamento Técnico sobre Seguridad de Presas y Embalses, España. (15) Triana, E. (2007). Contribución a la estimación de consecuencias de fallo y rotura de presas en el contexto del análisis de riesgos. Tesis doctoral, Universidad Politécnica de Valencia. Ignacio Escuder, PhD, MSc, Civil Engineer, is currently Professor at Universidad Politecnica de Valencia and permanent member of the Spanish National Committee on Large Dams. Manuel G. Membrillera, PhD, Civil Engineer, is lecturing at Universidad de Castilla la Mancha an involved in several dam safety projects at Universidad Politecnica de Valencia. Pedro Moreno, Civil Engineering at Confederacion Hidrografica del Duero, is responsible of several dams and joints the working group in charge of implementing a risk based complementary dam safety program. Oscar Perez, Civil Engineer at Ofiteco works on consultancy projects for different river basin authorities in Spain. Liana Ardiles, PhD, Civil Engineer, is currently Technical Director of the Confederacion Hidrografica del Duero, with more than 20 dams under her responsibility. Esther Jenaro, Civil Engineering at Confederacion Hidrografica del Duero, is responsible of several dams and joints the working group in charge of implementing a risk based complementary dam safety program. 14

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